In known molding machines, the plastic material is fed into a mold by a feed circuit connected to a bin storing plastic material in the form of pre-expanded granules of a given colour. The feed circuit normally comprises at least one reservoir, in which a batch of material drawn from the bin is maintained at a given pressure; and at least one, normally pneumatic, loader connected to the reservoir by a header to receive a quantity of material from the reservoir and feed it directly into the mold.
A change in product colour involves emptying the feed circuit, in particular the reservoir, of the previous batch of a first material; cleaning the feed circuit; and loading the reservoir with a batch of a different-coloured second material.
As described, for example, in DE 102004016756, the feed circuit is still normally cleaned by successively aspirating the first material from the reservoir and then from various points of the feed circuit downstream from the reservoir, to clear the machine of the first material batch.
This method has serious drawbacks, mainly due to the tendency of the pre-expanded granular material to charge electrostatically and adhere to the inside of the reservoir and the feed circuit conduits, so that successive suction cleaning fails to ensure all the material is removed. As a result, when the machine is turned on again, any granules of the previous colour left inside the feed circuit contaminate the colour the initial output of the machine, which must therefore be rejected.
The above considerations apply to DE-3900664, U.S. Pat. No. 5,961,734 and WO-2004/103522.